Item (Mage_Catalog_Model_Product) with the same id "xxx" already exist 

In file

lib/Varien/Data/Collection.php

find the following block

public function addItem(Varien_Object $item)
    
{
        $itemId 
$this->_getItemId($item);
        if (!
is_null($itemId)) {
            
if (isset($this->_items[$itemId])) {
              
  throw new Exception('Item ('.get_class($item).') with the same id "'.$item->getId().'" already exist');
            
else {
                $this
->_items[$itemId] $item;
            
}
        }
        
else {
            $this
->_items[] $item;
        
}
        
return $this;
    
}

 

and comment the line starting with throw new... like this:

public function addItem(Varien_Object $item)
    
{
        $itemId 
$this->_getItemId($item);
        if (!
is_null($itemId)) {
            
if (isset($this->_items[$itemId])) {
              
//  throw new Exception('Item ('.get_class($item).') with the same id "'.$item->getId().'" already exist');
            
else {
                $this
->_items[$itemId] $item;
            
}
        }
        
else {
            $this
->_items[] $item;
        
}
        
return $this;
    
}